Nonaqueous electrolyte secondary batteries, such as lithium secondary batteries, are being used as a wide variety of power sources, ranging from power sources for so-called portable electronic devices such as mobile phones and notebooks, to large stationary power sources as well as automotive power sources for driving, in automobiles or the like. However, the demands placed on the secondary batteries that are used have become ever more challenging in recent years, accompanying the higher performances of electronic devices and the growing use of secondary batteries as automotive power sources for driving and as large stationary power sources. It is now required that the characteristics of secondary batteries afford high battery performance levels in terms of, for instance, higher capacity, and improved high-temperature storage characteristics and cycle characteristics.
Ordinarily, the electrolyte solutions that are used in nonaqueous electrolyte secondary batteries are mainly made up of an electrolyte and a nonaqueous solvent. Examples of the main component of the nonaqueous solvent include, for instance, cyclic carbonates such as ethylene carbonate or propylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate; and cyclic carboxylic acid esters such as γ-butyrolactone, γ-valerolactone or the like.
As mentioned above, the characteristics of nonaqueous electrolyte secondary batteries, in particular nonaqueous electrolyte lithium secondary batteries, specifically higher capacity and improved high-temperature storage characteristic, cycle characteristic and so forth, call for ongoing improvement given the relentless demand for higher performance in secondary batteries in recent years.
The following methods for raising the capacity, for example, have been examined: pressing the active material layer of the electrode in order to reduce, as much as possible, the volume within the battery that is outside the material; broadening the utilization range of the positive electrode to support use to higher potentials. However, when the capacity is raised by pressing the active material layer of the electrode, it is then difficult to achieve uniformity for the active material and a portion of the lithium will precipitate due to nonuniform reactions and/or deterioration of the active material will be facilitated, and the ability to obtain satisfactory properties is thus readily impaired. When the positive electrode utilization range is broadened in support of use at higher potentials, the activity of the positive electrode undergoes an additional increase and an acceleration of the deterioration induced by reactions between the positive electrode and the electrolyte solution is then prone to occur.
Another problem brought about by reducing the void space within the battery in pursuit of higher capacities is that the internal pressure of the battery undergoes a substantial increase when even small amounts of gas are generated by degradation of the electrolyte solution. In particular, in almost all cases where a nonaqueous electrolyte secondary battery is used as a back-up power source for power outages or as a power source for portable devices, a weak current is supplied in order to compensate for battery self-discharge, thus establishing a state of constant discharge. Due, in such a continuous charging state, to heat generation by the device at the same time that the electrode active materials are continually in a highly active state, capacity deterioration by the battery is accelerated and gas generation due to degradation of the electrolyte solution is prone to occur. When large amounts of gas are generated, the safety valve ultimately operates in the case of a battery in which a safety valve operates when an abnormal increase, e.g., overcharging, is detected. For a battery not equipped with a safety valve, the battery may be swollen by the pressure of the generated gas and the battery itself may become unusable. These problems become even more severe when the nonaqueous electrolyte secondary battery is placed in a high-temperature environment.
For example, in the case of a nonaqueous electrolyte secondary battery that uses the electrolyte solution described in Patent Document 1, it is taught that excellent charge/discharge cycling characteristics are exhibited through the incorporation in the nonaqueous electrolyte of a specific compound having an ether linkage. However, this is still unsatisfactory in particular because the battery characteristics decline in high-temperature environments.
A compound containing an ether linkage has been introduced with the goal of improving the cycle characteristics of nonaqueous electrolyte secondary batteries (Patent Document 2). However, because ether linkage-containing compounds have a lower oxidative decomposition potential than carbonates and carboxylate esters (Non-patent Document 1), the problem here has been unsatisfactory high-temperature storage characteristics and unsatisfactory high-temperature continuous charging properties.